What is a full header?

Full headers are more than the "From" and "To" lines your email
client displays at the top of your email messages. The full
headers contain a lot of information, including every hop a
message has taken across the Internet to get from its sender
to you, its final recipient. This data of where the message
has actually come from can be very valuable in tracking down
the real origin of an email. Email can be forged fairly easily,
but every email will always be marked with the true IP address
of the sending computer. The trick is getting your email client
to display these lines so that you can analyze them yourself or
forward them to your support staff.